Limited-volume apparatus for forming thin film aerogels on semiconductor substrates

ABSTRACT

An apparatus and method for forming thin film aerogels on semiconductor substrates is disclosed. It has been found that in order to produce defect-free nanoporous dielectrics with a controllable high porosity, it is preferable to substantially limit evaporation and condensation of pore fluid in the wet gel thin film, e.g. during gelation, during aging, and at other points prior to obtaining a dried gel. The present invention simplifies the atmospheric control needed to prevent evaporation and condensation by restricting the atmosphere in contact with the wet gel thin film to an extremely small volume. In one embodiment, a substrate 26 is held between a substrate holder 36 and a parallel plate 22, such that a substantially sealed chamber 32 exists between substrate surface 28 and chamber surface 30. Preferably, the average clearance between surfaces 28 and 30 is less than 5 mm, or more preferably, less than 1 mm. Temperature control means 34 may optionally be used to control the temperature in chamber 32. In operation, the atmosphere in chamber 32 becomes saturated by an extremely small amount of pore fluid evaporated from a wet gel thin film on surface 28, thus preventing further evaporation or condensation. This invention is ideally suited for rapid aging of thin film wet gels.

This application is a divisional of application Ser. No. 08/746,697filed Nov. 14, 1996, which claims priority from provisional applications60/006,852, filed Nov. 16, 1995; 60/006,853, filed Nov. 16, 1995;60/012,764, filed Mar. 4, 1996; 60/014,005 filed Mar. 25, 1996;60/012,800, filed Mar. 4, 1996; and 60/022,842, filed Jul. 31, 1996.

FIELD OF THE INVENTION

This invention pertains generally to deposition tools and methods forforming porous thin films on semiconductor substrates, and morespecifically to tools and methods for aging and drying wet gel thinfilms during the fabrication of nanoporous dielectrics.

BACKGROUND OF THE INVENTION

Nanoporous dielectrics are some of the most promising new materials forsemiconductor fabrication. These dielectric materials contain a solidstructure, for example of silica, which is permeated with aninterconnected network of pores having diameters typically on the orderof a few nanometers. These materials may be formed with extremely highporosities, with corresponding dielectric constants typically less thanhalf the dielectric constant of dense silica. And yet despite their highporosity, it has been found that nanoporous dielectrics may befabricated which have high strength and excellent comparability withmost existing semiconductor fabrication processes. Thus nanoporousdielectrics offer a viable low-dielectric constant replacement forcommon semiconductor dielectrics such as dense silica.

The preferred method for forming nanoporous dielectrics is through theuse of sol-gel techniques. The word sol-gel does not describe a productbut a reaction mechanism whereby a sol, which is a colloidal suspensionof solid particles in a liquid, transforms into a gel due to growth andinterconnection of the solid particles. One theory is that throughcontinued reactions within the sol, one or more molecules in the sol mayeventually reach macroscopic dimensions so that it/they form a solidnetwork which extends substantially throughout the sol. At this point(called the gel point), the substance is said to be a gel By thisdefinition, a gel is a substance that contains a continuous solidskeleton enclosing a continuous liquid phase. As the skeleton is porous,the term "gel" as used herein means an open-pored solid structureenclosing a pore fluid.

One method of forming a sol is through hydrolysis and condensationreactions, which can cause a multifunctional monomer in a solution topolymerize into relatively large, highly branched particles. Manymonomers suitable for such polymerization are metal alkoxides. Forexample, a tetraethylorthosilicate (TEOS) monomer may be partiallyhydrolyzed in water by the reaction

    Si(OEt).sub.4 +H.sub.2 O→HO--Si(OEt).sub.3 +EtOH

Reaction conditions may be controlled such that, on the average, eachmonomer undergoes a desired number of hydrolysis reactions to partiallyor fully hydrolyze the monomer. TEOS which has been fully hydrolyzedbecomes Si(OH)₄. Once a molecule has been at least partially hydrolyzed,two molecules can then link together in a condensation reaction, such as

    (OEt).sub.3 Si--OH+HO--Si(OH).sub.3 →(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O

or

    (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3 →(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH

to form an oligomer and liberate a molecule of water or ethanol. TheSi--O--Si configuration in the oligomer formed by these reactions hasthree sites available at each end for further hydrolysis andcondensation. Thus, additional monomers or oligomers can be added tothis molecule in a somewhat random fashion to create a highly branchedpolymeric molecule from literally thousands of monomers. An oligomerizedmetal alkoxide, as defined herein, comprises molecules formed from atleast two alkoxide monomers, but does not comprise a gel.

Sol-gel reactions forms the basis for xerogel and aerogel filmdeposition. In a typical thin film xerogel process, an ungelledprecursor sol may be applied (e.g., spray coated, dip-coated, orspin-coated) to a substrate to form a thin film on the order of severalmicrons or less in thickness, gelled, and dried. The precursor sol oftencomprises a stock solution and a solvent, and possibly also a gelationcatalyst that modifies the pH of the precursor sol in order to speedgelation. During and after coating, the volatile components in the solthin film are usually allowed to rapidly evaporate. Thus, thedeposition, gelation, and drying phases may take place simultaneously(at least to some degree) as the film collapses rapidly to a dense film.In contrast, an aerogel process differs from a xerogel process largelyby avoiding pore collapse during drying of the wet gel. Some methods foravoiding pore collapse include wet gel treatment withcondensation-inhibiting modifying agents (as described in U.S. Pat. No.5,470,802, A Low Dielectric Constant Material For ElectronicsApplications, issued on Nov. 28, 1995 to Gnade, Cho and Smith),supercritical pore fluid extraction, and freeze-drying.

Aerogels are the preferable of the two dried gel materials forsemiconductor thin film dielectric applications. Typical thin filmxerogel methods produce films having limited porosity (up to 60% withlarge pore sizes, but generally substantially less than 50% with poresizes useful in submicron semiconductor fabrication). An aerogel thinfilm, on the other hand, may be formed with almost any desired porositycoupled with a very fine pore size. Preferably, for semiconductorapplications these nanoporous materials have average pore sizes lessthan 50 nm (and more preferably less than 10 nanometers and still morepreferably less than 5 nanometers). The nanoporous inorganic dielectricsinclude the nanoporous metal oxides, particularly nanoporous silica.

SUMMARY OF THE INVENTION

It has now been found that in the production of nanoporous dielectricsit is preferable to subject the wet gel thin film to a process known asaging. Hydrolysis and condensation reactions do not stop at the gelpoint, but continue to restructure, or age, the gel until the reactionsare purposely halted. It is believed that during aging, preferentialdissolution and redeposition of portions of the solid structure producesbeneficial results, including higher strength, greater uniformity ofpore size, and a greater ability to resist pore collapse during drying.However, aging a wet gel in thin film form is difficult, as the filmcontains an extremely small amount of pore fluid that must be heldfairly constant for a period of time in order for aging to occur. Ifpore fluid evaporates from the film before aging has strengthened thenetwork, the film will tend to densify in xerogel fashion. On the otherhand, if excess pore fluid condenses from the atmosphere onto the thinfilm before the network has been strengthened, this may locally disruptthe aging process and cause film defects.

It has now been found that some method of pore fluid evaporation ratecontrol during aging is beneficial to aerogel thin film fabrication. Inprinciple, evaporation rate control during aging can be accomplished byactively controlling the pore fluid vapor concentration above the wafer.However, the total amount of pore fluid contained in, for instance, a 1μm thick 70% porous wet gel deposited on a 150 mm wafer is only about0.012 ml, an amount that would easily fit in a single 3 mm diameter dropof fluid. Actively controlling the pore fluid vapor concentration (byadding or removing solvent to the atmosphere) to allow no more than,e.g., 1% pore fluid evaporation during aging presents a difficultproposition; the surface area of the thin film is high and the allowabletolerance for pore fluid variations is extremely small. In particular,evaporation and condensation control are especially important for rapidaging at elevated temperature, where production processes haveheretofore apparently not been practically possible.

The present invention, in its simplest form, overcomes the evaporationrate control problem by not attempting to actively control pore fluidvapor concentration above a wafer at all; instead, the wafer isprocessed in an extremely low-volume chamber, such that through naturalevaporation of a relatively small amount of the pore fluid contained inthe wet gel film, the processing atmosphere becomes substantiallysaturated in pore fluid. Unless the wafer is cooled at some point in asubstantially saturated processing atmosphere, this method alsonaturally avoids problems with condensation, which should generally beavoided, particularly during high temperature processing.

In accordance with the present invention, a method is presented forpost-deposition processing (e.g. gelation, aging, and/or drying) of awet gel thin film deposited on a semiconductor substrate. This methodcomprises the step of placing a substrate in a preferably substantiallysealed chamber, where the substrate has a wet gel thin film, wetted byat least a first pore fluid, deposited thereon. The first pore fluidpreferably principally comprises a polyol. The chamber is preferably notpressurized or uses at most a moderate overpressure such that there islittle or no leakage, and atmospheric flow is generally avoided. Thismethod further generally utilizes controlling the chamber at atemperature selected in the range of between 25° C. and 200° C. (andmore preferably between 80° C. and 150° C.), with the temperatureselected such that less than 5% (preferably less than 1%, and morepreferably less than 0.5%) of the first pore fluid contained in the wetgel thin film is required to substantially saturate the atmosphere inthe chamber.

Preferably, the step of controlling the chamber temperature comprisesramping the chamber to a specified temperature and holding the chamberat that temperature for a time period, e.g. such that the film is aged.After sufficient aging, the fluid can be evaporated.

In another aspect of the invention, an apparatus for processing a sol orwet gel thin film deposited on a substrate is disclosed. This apparatuscomprises a body capable of substantially enclosing at least a firstregion of a substrate surface, where the first region has a sol or wetgel thin film deposited thereon. The body preferably has a chambersurface capable of being positioned substantially adjacent to the thinfilm without contacting the thin film, thus forming an extremelylow-volume atmospheric chamber adjacent the thin film. The apparatusalso preferably comprises some means for controlling the temperaturewithin the chamber.

The low-volume chamber may be designed to have a volume relative to thevolume of the wet gel film to be processed; e.g., generally less than orequal to 5000 times the volume of the thin film in the first region, ormore preferably less than or equal to 1000 times greater, or still morepreferably less than 500 times greater. Alternately, the chamber volumemay be designed with a specific pore fluid, pore fluid volume andprocessing temperature in mind, such that less than a specifiedpercentage of the pore fluid is required to substantially saturate theatmospheric chamber. Some specific designs have chamber surfaces whichmay be positioned to within 5 mm or even to within 1 mm or less of thethin film.

Dried gels produced with this simple thin film aerogel fabricationprocess and apparatus can be used in many applications. Some of theseuses may not have been cost effective using prior art methods. Theseuses include low dielectric constant thin films (particularly onsemiconductor substrates), miniaturized chemical sensors, thermalisolation structures, and thermal isolation layers (including thermalisolation structures for infrared detectors). As a general rule, manylow dielectric constant thin films prefer porosities greater than 60%,with critical applications preferring porosities greater than 80 or 90%,thus giving a substantial reduction in dielectric constant. However,structural strength and integrity considerations may limit the practicalporosity to no more than 90%. Some applications, including thermalisolation structures and thermal isolation layers, may need to sacrificesome porosity for higher strength and stiffness.

The term "thin film" as used herein refers to a film having an averagethickness of less than 2 microns. It should be noted that the presentinvention is generally not applicable to bulk gel processing, whereevaporation poses substantially different problems. For example, porefluid evaporation during aging is generally not a problem in bulk gelprocessing, since the ratio of surface area to fluid volume may be fouror five orders of magnitude less than the same ratio for a thin film. Bythe same token, however, high-viscosity, low-vapor pressure pore fluids,preferred for thin film applications, are extremely difficult to dry orsolvent exchange from a bulk gel, and are generally not used in bulk gelapplications.

BRIEF DESCRIPTION OF THE DRAWING

The present invention, including various features and advantagesthereof, may be best understood with reference to the following drawing,wherein:

FIG. 1 contains a graph of the variation of evaporation rate withsaturation ratio and solvent type;

FIG. 2 contains a graph of the change in gel times (without solventevaporation) for bulk ethylene glycol-based gels as a function of basecatalyst;

FIG. 3 contains a graph of the variation of modulus with density for anon-glycol-based gel and an ethylene glycol-based gel;

FIG. 4 contains a graph of the viscosity variation as a function ofalcohol volume fraction for ethylene glycol/alcohol mixtures;

FIG. 5 contains a graph of the evaporation rate for ethylene glycol as afunction of temperature and atmospheric saturation ratio;

FIG. 6 contains a graph of the variation in refractive index duringprocessing for a film produced using a 60/40 ethylene glycol/ethanolsolution with a substantially uncontrolled atmosphere;

FIG. 7 contains a graph of the theoretical relationship betweenporosity, refractive index, and dielectric constant for nanoporoussilica dielectrics;

FIGS. 8A-8B contain cross-sections of a semiconductor substrate atseveral points during deposition of a thin film which may be fabricatedusing the present invention;

FIG. 9 is a flow chart of a deposition process for a nanoporousdielectric which utilizes the present invention;

FIG. 10 contains a graph of the theoretical molar ratio of first solventmolecules to metal atoms vs. porosity of a nanoporous dielectric whichmay be fabricated using the present invention;

FIG. 11 contains a graph of the evaporation rate for glycerol a functionof temperature and atmospheric saturation ratio;

FIG. 12 contains a graph showing change in vapor pressure withtemperature;

FIG. 13 contains a graph showing the shrinkage of a thin film when agedin a substantially closed chamber having a 5 mm clearance with the thinfilm;

FIG. 14 contains a graph showing the shrinkage of a thin film when agedin a substantially closed chamber having a 1 mm clearance with the thinfilm;

FIG. 15 contains a graph of the theoretical molar ratio of glycerolmolecules to metal oxide molecules vs. porosity of a nanoporousdielectric;

FIGS. 16A and 16B contain, respectively, a cross-sectional and a planview of a sol-gel thin film processing apparatus according to thepresent invention;

FIG. 16C contains a cross-sectional view of the same apparatus incontact with a substrate;

FIGS. 17A and 17B contain, respectively, cross-sectional views ofanother apparatus according to the present invention, empty andenclosing a substrate;

FIGS. 18A and 18B contain, respectively, cross-sectional views of yetanother apparatus according to the present invention, empty andenclosing a substrate; and

FIGS. 19A, 19B and 19C contain cross-sectional views of additionalapparatus configurations which illustrate other aspects of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several recent advances have resulted in more commercially viable agingprocesses for aerogel thin films. These include multi-solvent solprecursors, low-volatility polyol-based sol precursors, and rapid agingtechniques. For example, in one preferred system, all three of theseimprovements are combined. A multi-solvent sol precursor comprises afirst polyol solvent having a low volatility, which is used to controlgel density and pore fluid evaporation, and a second solvent (e.g.ethanol) having a low viscosity and a relatively high volatility, whichis used to control pre-deposition reaction rate and to allow uniformcoating. Preferably the low volatility solvent is one with a boilingpoint in the 175-300° C. range and (for TEOS based gels) is misciblewith both water and ethanol. Some suitable polyols are trihydricalcohols, such as glycerol, and dihydric alcohols (glycols), such asethylene glycol, 1,4-butylene glycol, and 1,5-pentanediol. The lowvolatility solvent is preferably principally ("principally" as usedherein means 50% or greater by volume) comprises the aging fluid in thissystem, such that a wet gel thin film may be aged at elevatedtemperature for a relatively short time period.

Even with such a system, it has now been found to be desirable to atleast age a wet gel thin film under controlled atmospheric conditions.In an effort to better understand solvent evaporation from thin film wetgels, we have modeled isothermal solvent vaporization from a wafer as afunction of percent saturation. This modeling is based on basic masstransfer theory. Transport Phenomena, (particularly Chapters 16 and 17)by R. B. Bird, W. E. Stewart, and E. N. Lightfoot, is a good referencefor mass transfer theory. Calculations were performed for a range ofsolvents, and the ambient temperature evaporation rates for some ofthese solvents are given in FIG. 1.

For evaporation to not be a processing problem, the product of theevaporation rate and processing time (preferably on the order ofminutes) must be significantly less than the film thickness. Thissuggests that for solvents such as ethanol, the atmosphere above thewafer would have to be maintained at over 99% saturation. However, therecan be problems associated with allowing the atmosphere to reachsaturation or supersaturation. Some of these problems are related tocondensation of an atmospheric constituent upon the thin film.Condensation on either the gelled or ungelled thin film has been foundto cause defects in an insufficiently aged film. Thus, it is generallydesirable to avoid actively controlling the atmosphere's degree ofsolvent saturation.

The use of a polyol allows a loosening (as compared to prior artsolvents) of the required atmospheric control during deposition and/orgelation. This is because that even though supersaturation should stillgenerally be avoided, the atmospheric solvent concentration need not becontrolled to avoid excessive evaporation. FIG. 5 shows how theevaporation rate of ethylene glycol varies with temperature andatmospheric solvent concentration. FIG. 11 shows how the evaporationrate of glycerol varies with temperature and atmospheric solventconcentration. It has been our experience that, with polyols, acceptablegels can be formed by depositing and gelling in an uncontrolled or asubstantially uncontrolled atmosphere ("uncontrolled" is used herein tomean without actively controlling the solvent concentration in theatmosphere). In the most preferred approach, deposition and gelationtypically require only standard cleanroom temperature and humiditycontrols, although the wafer and/or precursor sol may have independenttemperature controls.

One attractive feature of using a polyol as a solvent is that at ambienttemperature, the evaporation rate is sufficiently low so that severalseconds at ambient conditions will not yield dramatic shrinkage for thinfilms. However, in addition to serving as a low vapor pressure andwater-miscible solvent, polyols may also participate in sol-gelreactions. Although the exact reactions in this process have not beenfully studied, some reactions can be predicted. If tetraethoxysilane(TEOS) is employed as a precursor with an ethylene glycol solvent, theethylene glycol can exchange with the ethoxy groups:

    Si(OC.sub.2 H.sub.5).sub.4 +x HOC.sub.2 H.sub.4 OH←Si(OC.sub.2 H.sub.5).sub.4-x (OC.sub.2 H.sub.4 OH).sub.x +x C.sub.2 H.sub.5 OH

Similarly, if tetraethoxysilane (TEOS) is employed as a precursor with aglycerol solvent, the glycerol can exchange with the ethoxy groups:

    Si(OC.sub.2 H.sub.5).sub.4 +x [HOCH.sub.2 CH(OH)CH.sub.2 OH]←Si(OC.sub.2 H.sub.5).sub.4-x [OC.sub.3 H.sub.5 (OH).sub.2 ].sub.x +x [C.sub.2 H.sub.5 OH]

In principle, the presence and concentration of these chemical groupscan change the precursor reactivity (i.e., gel time), modify the gelmicrostructure (surface area, pore size distribution, etc.), change theaging characteristics, or change nearly any other characteristic of thegel.

Ethylene glycol and glycerol may react with TEOS and produce a dried gelwith surprisingly different properties than that of an ethanol/TEOS gel.Unanticipated property changes in the ethylene glycol/TEOS based gelsand the glycerol/TEOS based gels generally include (at least on mostformulations):

Lower density, achievable without supercritical drying or pre-dryingsurface modification

Shorter gel times at a given catalyst content

Strengths of bulk samples which are approximately an order of magnitudegreater (at a given density) than conventional TEOS gels

Very high surface area (˜1,000 m² /g)

High optical clarity of bulk samples (likely due to a narrower pore sizedistribution than conventional TEOS gels)

Low Density

With polyol solvent systems, it is possible to form dried gels at verylow densities without pre-drying surface modification or supercriticaldrying. These low densities can generally be down around 0.3 to 0.2g/cm³ (non-porous SiO₂ has a density of 2.2 g/cm³), or with care, around0.1 g/cm³. Stated in terms of porosity (porosity is the percentage of astructure which is hollow), this denotes porosities of about 86% and 91%(about 95% porosity with a density of 0.1 g/cm³). As shown in FIG. 7,these porosities correspond to dielectric constants of about 1.4 for the86% porous, and 1.2 for 91% porous. The actual mechanism that allowsthese high porosities is not fully known. However, it may be because thegels have high mechanical strength, because the gels do not have as manysurface OH (hydroxyl) groups, a combination of these, or some otherfactors. This method also obtains excellent uniformity across the wafer.FIG. 6 shows the refractive index (and thus generally the porosity) atseveral locations on a sample semiconductor substrate.

If desired, this process can be adjusted (by varying the TEOS/solventratios) to give any porosity from above 90% down to about 20%, or even10%. Typical prior art dried gels with small pore sizes required eithersupercritical drying or a surface modification step before drying toachieve these low densities. While some prior art xerogels haveporosities greater than 50%, these prior art xerogels had substantiallylarger pore sizes (typically above 100 nm). These large pore size gelshave less mechanical strength. Additionally, their large pore size makesthem unsuitable for filling small (typically less than 1 μm) patternedgaps on a microcircuit.

Density Prediction

By varying the ratio of ethylene glycol (EG) to ethanol (EtOH) in theprecursor (at a fixed silica content), the density after ethanol/waterevaporation can be calculated. This is likely due to the low volatilityof the second solvent. To the extent that further shrinkage is preventedduring aging and drying, this allows prediction of the density (and thusporosity) of the dried gel. Although this density prediction hadgenerally not been a large problem with bulk gels, thin film gels hadtypically needed excellent atmospheric controls to enable consistentdensity predictions. Table 1 shows the predicted and actual density forthree different EG/EtOH ratios after substantial ethanol and waterremoval, but before drying (EG removal).

                  TABLE 1                                                         ______________________________________                                        Correlation between predicted and                                               measured density of wet bulk gels after                                       ethanol/water evaporation.                                                                               Measured                                            Predicted Density (g/cm.sup.3)                                                Density after drying @                                                       Stock Solution (g/cm.sup.3) 80° C.                                   ______________________________________                                        40% EtOH/60% EG 0.37     0.40                                                   51% EtOH/49% EG 0.43 0.45                                                     60% EtOH/40% EG 0.53 0.50                                                   ______________________________________                                    

To some degree, the glycerol-based processes behave similarly to theethylene glycol-based processes. The glycerol-based gels havedramatically lower evaporation and shrinkage rates during aging. Thisallows atmospheric control to be loosened during aging. We havefabricated acceptable glycerol-based gels with no atmospheric controlsduring aging.

Shorter Gel Times

In addition to enabling prediction of the density, the use of polyolsmay also change other properties of the sol-gel process. FIG. 2 showsgel times for two different ethylene glycol-based compositions as afunction of the amount of ammonia catalyst used. These gel times are forbulk gels for which there is no evaporation of ethanol and/or water asthere would be for thin films. Evaporation increases the silica contentand thus decreases the gel time. Therefore, these gels times may be theupper limit for a given precursor/catalyst. The gel times reported inFIG. 2 are approximately an order of magnitude shorter than precursorswithout a polyol. Gel times are generally also a first order dependenceon the concentration of ammonia catalyst. This implies that it may bepossible to easily control the gel times. For thin films of these newpolyol-based gels, it is routine to obtain gelation within minutes, evenwithout a gelation catalyst.

Higher Strength

The properties of the polyol-based samples appear to be quite differentfrom regular gels as evidenced by both a low degree of drying shrinkageand differences in qualitative handling of the wet and dry gels. Thus,upon physical inspection, both the glycerol-based and ethyleneglycol-based dried gels seem to have improved mechanical properties ascompared to conventional dried gels. FIG. 3 shows the bulk modulusmeasured during isostatic compaction measurements of one sample preparedusing one ethylene glycol-based and one conventional ethanol-based driedbulk gel (both have the same initial density). After initial changesattributed to buckling of the structure, both samples exhibit power lawdependence of modulus with density. This power law dependence is usuallyobserved in dried gels. However, what is surprising is the strength ofthe ethylene glycol-based dried gel. At a given density (and thus,dielectric constant), the modulus of the ethylene glycol based gel is anorder of magnitude higher than the conventional precursor gel. Theglycerol-based gels also seem to have a high strength; generally, thestrength is at least as good as the ethylene glycol-based gels. Thereasons for this strength increase are unclear but correspondingly maybe related to the very high surface area of these dried gels (>1,000 m²/g) and the seemingly narrow pore size distribution.

High Surface Area

We measured the surface areas of some dried bulk gels. These surfaceareas were on the order of 1,000 m² /g, as compared to our typical driedgels which have surface area in the 600-800 m² /g range. These highersurface areas imply smaller pore size which may lead to improvedmechanical properties. It is unclear at this time why these highersurface areas are obtained with the polyol-based dried gels.

Pore Size Distribution

The optical clarity of these dried bulk gels was greater than any driedgels at this density that we have previously made. It is possible thatthis excellent optical clarity is due to a very narrow pore sizedistribution. However, it is unclear why the polyols have this effect.It is still not clear whether the apparently narrow pore sizedistribution is a result of a different microstructure at the gelationstage or differences in aging. Preliminary measurements on a bulk gelsample (with a density of about 0.22 g/cm³) showed that the mean porediameter was 16.8 nm.

As shown above, some properties of the polyol-based gels apply to bothbulk gels and thin films. However, some advantages are most evident whenapplied to thin films, such as nanoporous dielectric films onsemiconductor wafers. One important advantage is that this new methodallows high quality nanoporous films to be processed with no atmosphericcontrols during deposition or gelation.

Although it is important to be able to deposit and gel thin nanoporousfilms without atmospheric controls, it is also desirable to age thinnanoporous films without complicated atmospheric controls. It has beendiscovered that this presents a bigger challenge than deposition. Theprimary reason is that while deposition and room temperature gelationcan take place in minutes, or even seconds; room temperature agingtypically requires many hours. Thus, an evaporation rate that providesacceptable shrinkage for a short process, may cause unacceptableshrinkage when the process times are lengthened by an order of magnitude(and/or if processing temperatures are raised).

As an example, we have found that with some polyol-based gels, such asthe ethylene glycol- and glycerol-based gels, a satisfactory aging timeat room temperature is on the order of a day. However, Table 2 showsthat, by using higher temperatures, we can age with times on the orderof minutes. Thus, when these times and temperatures are combined withthe evaporation rates of FIG. 1, FIG. 5, and FIG. 11, they give theapproximate thickness loss during aging as shown in Table S. Theseestimated thickness losses need to be compared with acceptable thicknesslosses. While no firm guidelines for acceptable thickness loss exist,one proposed guideline, for some microcircuit applications such asnanoporous dielectrics, is that the thickness losses should be less than2% of the film thickness. For a hypothetical nominal film thickness of 1μm (actual film thicknesses may typically vary from significantly lessthan 0.5 μm to several 1 μm thick), this gives an allowable thicknessloss of 20 nm. As shown in Table 3, the glycerol-based gels (and otherpolyol-based gels with low vapor pressures) can achieve this preliminarygoal without atmospheric control at room temperature. However, othersolvent systems and/or higher temperature aging require at least somedegree of atmospheric control.

                  TABLE 2                                                         ______________________________________                                        Approximate Aging Time as a Function of Temperature                             For Some Polyol-Based Gels                                                      Aging Temperature                                                                          Aging Time For Polyol-Based Gels                               (Degrees C.) (Order of Magnitude Approximations)                            ______________________________________                                         25          1            day                                                   100 5 minutes                                                                 140 1 minute                                                                ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________    Approximate Thickness Loss During Aging vs. Saturation Ratio.                       Thickness Loss During Aging                                                   Ethanol-Based Gel                                                                       EG-Based Gel                                                                              Glycerol-Based Gel                                  Aging Time/ % Saturation % Saturation % Saturation                          Temperature                                                                         0% 50%                                                                              99% 0%  50% 99% 0%  50% 99%                                       __________________________________________________________________________    1 day/                                                                              8 mm                                                                             7 mm                                                                             86 μm                                                                          17 μm                                                                            7 μm                                                                         172 nm                                                                             13 nm                                                                             5 nm                                                                              .1 nm                                      25° C.                                                                 300 sec/ -- -- --  3 μm 1.2 μm  90 nm 600 nm 420 nm   9 nm                                                 100° C.                             60 sec/ -- -- -- -- -- --  6 μm  3 μm  60 nm                            140° C.                                                              __________________________________________________________________________

One disadvantage of polyols, especially trihydric alcohols and otherhigher viscosity polyols, are their relatively high viscosities whichcould cause problems with gap-filling and/or planarization. As describedin copending U.S. patent application serial # TBD (Attorney's DocketTI-21623), titled Aerogel Thin Film Formation From Multi-SolventSystems, by Smith et al., a low viscosity, high volatility solvent canbe used to lower the viscosity. FIG. 4 shows the calculated viscosity ofsome ethylene glycol/alcohol and glycerol/alcohol mixtures at roomtemperature. As the figure shows, a small quantity of alcoholsignificantly reduces the viscosity of these mixtures. Also, if theviscosity using 10 ethanol in the stock solution is higher than desired,further improvement can be realized by employing methanol andtetramethoxysilane in the precursor solution. The viscosities reportedin FIG. 4 are for pure fluid mixtures only. In fact, depending upon thefilm precursor solution, the precursor solution might contain glycerol,alcohol, water, acid and partially reacted metal alkoxides. Afterrefluxing, but before catalysis, the measured viscosity as a function ofethylene glycol content is shown in Table 4. As predicted, the use ofmethanol significantly lowers the viscosity. Of course, the viscositycan be increased before deposition by catalyzing the condensationreaction and hence, the values reported in Table 4 represent lowerbounds.

                  TABLE 4                                                         ______________________________________                                        Measured Viscosity and Density of Glycol-Based                                  Stock Solutions Before Activation.                                                                   Viscosity                                                                              Viscosity                                      @25° C. @40° C.                                                Stock Solution "Solvent" (cp) (cp)                                          ______________________________________                                        100% EtOH            1.5      --                                                40% Ethylene glycol/60% EtOH 3.1 --                                           49% Ethylene glycol/51% EtOH 4.0 --                                           60% Ethylene glycol/40% EtOH 5.4 --                                           40% Ethylene glycol/60% Methanol 1.6 --                                       100% Ethylene glycol 11.0 7.8                                                 40% Glycerol/60% EtOH 5.8 --                                                  50% Glycerol/50% EtOH 9.0 --                                                  60% Glycerol/40% EtOH 15.5 --                                                 100% Glycerol 1000. 7.8                                                     ______________________________________                                    

This multi-solvent approach may be combined with or replaced with analternative approach. This alternate approach uses elevated temperaturesto reduce the sol viscosity during application. For example, themeasured viscosity of the TEOS/ethylene glycol/water/nitric acidprecursor described in the second preferred embodiment is 11 centipoise(cp) at 25 degrees C., but only 7.8 cp at 40 degrees C. Thus by heatingand/or diluting the precursor during deposition, (such as by heating thetransfer line and deposition nozzle of a wafer spin station) theviscosity of the precursor sol can be lowered to nearly any givenviscosity. Not only does this preheat lower the sol viscosity, it mayalso speed gel times and accelerate the evaporation of any highvolatility solvents. It may also be desirable to preheat the wafer. Thiswafer preheat should improve process control and may improve gap fill,particularly for the more viscous precursors. However, for manyapplications, wafer preheat is not required, thus simplifying processflows. When using a spin-on application method with this no waferpreheat approach, the spin station would not require a temperaturecontrolled spinner.

The present invention allows high boiling point solvent gel processingat elevated temperatures, with acceptable shrinkage, by substantiallyenclosing the gel thin film in a relatively small, substantially closedchamber, at least during high-temperature aging. In operation, whateverevaporation does occur from the wafer raises the solvent saturationratio of the atmosphere inside the closed chamber. At any giventemperature, this evaporation continues until the partial pressure ofthe vapor increases enough to equal the vapor pressure of the liquid.Thus, solvent/temperature combinations with lower vapor pressure solventwill not allow as much liquid solvent to evaporate as a higher vaporpressure solvent combination allows. FIG. 12 shows how vapor pressurevaries with temperature for several solvents. If the chamber size isknown, the amount of evaporation can be calculated. FIG. 13 shows anestimate of how thick of a layer of solvent could potentially beevaporated from a 70% porous gel placed in a closed chamber with a 5 mmhigh airspace above the wafer. FIG. 14 shows a similar estimate for achamber with a 1 mm high airspace above the wafer. These figures showthat, with a 5 mm high airspace, the 20 nm preliminary goal is feasibleup to 50° C. for ethylene glycol-based gels and up to 120° C. forglycerol-based gels. With the 1 mm airspace, the 20 nm goal is feasibleup to 80° C. for the ethylene glycol-based gels and 150° C. for theglycerol-based gels. Of course, lower temperature processing allows lessevaporation. Volumetric evaporation control using the 1 mm chambersallows correspondingly less than 1 nm of thickness loss for bothethylene glycol-based and glycerol-based gels at 20° C.

One embodiment of the present invention is illustrated in FIGS. 16A, 16Band 16C. In this embodiment, a processing apparatus comprises a body 20,having a substantially planar plate 22 with a resilient seal 24 attachedthereto. Plate 22 need only be planar to the extent necessary to provideclearance with a thin film during operation, and may be constructed ofany material compatible with semiconductor fabrication, althoughmaterials with high thermal conductivity, such as stainless steel,glass, or aluminum are preferred. Resilient seal 24 should preferably bedesigned to withstand wet gel processing temperatures and pore fluids;many suitable TEFLON- or neoprene-based materials are known to thosewith ordinary skill in the art. Depending on the nature of temperaturecontrol used in the apparatus, it may be preferable to have seal 24 beeither substantially thermally insulating or thermally conductive.

In operation, body 20 may simply be rested on a substrate 26, as shownin FIG. 16C. In this embodiment, seal 24 functions both as anatmospheric seal and as a spacer which sets the volume of chamber 32formed by substrate surface 28, chamber surface 30 and seal 24. Forexample, seal 24 may be designed to compress to a thickness of about 1mm under the weight of plate 22, thus creating chamber 32 with a 1 mmheight when body 20 is placed on substrate 26. For many thin filmapplications, chamber 32 need only be substantially sealed, as somesmall degree of vapor leakage over the course of processing substrate 26will not appreciably affect the final film properties.

Body 20 finds application at many points in an aerogel thin filmprocess. It may be used to limit evaporation before a sol film hasgelled, as an aging chamber for wet gel thin films, as a storage ortransport chamber for such films, or as a drying chamber. In all ofthese applications, it is recognized that both sol and gel thin filmscontain extremely small amounts of liquid, such that a chamber oflimited volume is necessary to prevent substantial evaporation from thefilm.

In another embodiment, body 20 may comprise more elements, as shown inFIGS. 17A and 17B. In this embodiment, body 20 additionally comprises asubstrate holder 36 and substrate temperature control means 34. Thisembodiment shows the additional aspect of a seal 24 located outboard ofthe substrate (or in some cases seal 24 may even be deleted), such thata thin film may be formed on the entirety of substrate surface 28. Whenchamber 32 is closed, planar plate 22 and wafer holder 36 may bethermally coupled such that temperature control means 34 may be used tosimultaneously regulate the temperature of body 20, substrate 26 andchamber 32.

In another embodiment shown in FIGS. 18A and 18B, seal 24 provides somedegree of thermal isolation between planar plate 22 and wafer holder 36.This allows temperature control means 34 to control substratetemperature, while separate temperature control means 38 are used tocontrol planar plate temperature. Such an embodiment may have anadvantage for drying a wet gel film, as the temperature of planar plate22 can be selectively lowered to promote condensation on chamber surface30.

FIGS. 19A, 19B and 19C show additional aspects of the invention. Forexample, in FIG. 19A, substrate 26 is shown being processed in aninverted position. In this embodiment, accidental or purposefulcondensation onto chamber surface 30 may be collected without thepossibility of such condensation dropping onto substrate surface 26. InFIG. 19B, not only is substrate 26 processed inverted, but a firstsolvent layer 42 (preferably of the same composition as at least onepore fluid) is dispensed, e.g. from a first solvent supply tube 40, ontochamber surface 30 prior to closing the chamber. In this embodiment,layer 42 may be used to help saturate the processing atmosphere,resulting in less evaporation of pore fluid from substrate 26.

In FIG. 19C, an embodiment is shown wherein some atmospheric adjustmentmeans 44 is connected through at least one port 46 (which may becloseable) to chamber 32. Atmospheric adjustment means 44 may be used tocreate a vacuum or to overpressure chamber 32 as appropriate, or toexchange the atmosphere in chamber 32, or to supply a pore fluid vaporto chamber 32. This embodiment may be used, for example, to age a thinfilm at a temperature higher than the boiling point of a pore fluid, byoperating chamber 32 at above atmospheric pressure. This embodiment mayalso be used to remove at least a portion of the pore fluid vapor fromchamber 32 after aging, thereby allowing the thin film to dry.

Also in accordance with the present invention, several preferredembodiments for thin film nanoporous dielectric deposition methods arepresented herein. Referring now to FIG. 8A, a semiconductor substrate 26(typically in wafer form) is shown. Common substrates include silicon,germanium, and gallium arsenide, and the substrate may include activedevices, lower level wiring and insulation layers, and many other commonstructures not shown but known to those skilled in the art. Severalpatterned conductors 25 (e.g., of an Al-0.5% Cu composition) are shownon substrate 26. Conductors 25 typically run parallel for at least partof their length, such that they are separated by gaps 27 of apredetermined width (typically a fraction of a micron). Both theconductors and gaps may have height-to-width ratios much greater thanshown, with larger ratios typically found in devices with smallerfeature sizes.

In accordance with a first nanoporous dielectric method, 61.0 mLtetraethylorthosilicate (TEOS), 61.0 mL glycerol, 4.87 mL water, and 0.2mL 1M HNO₃ are mixed and refluxed for 1.5 hours at 60° C. After acooling period, the solution may be diluted down with ethanol to acomposition of 80% (by volume) original stock solution and 20% (byvolume) ethanol, thus reducing the viscosity. This is mixed vigorouslyand typically stored in a refrigerator at 7° C. to maintain stabilityuntil use. The solution is warmed to room temperature prior to filmdeposition. 3-5 mL of this precursor sol may be dispensed at roomtemperature onto substrate 26, which is then spun at 1500 to 5000 rpm(depending on desired film thickness) for about 5-10 seconds to form solthin film 29. The deposition can be performed in an atmosphere that hasno special control of solvent saturation (e.g., in a cleanroom withstandard humidity controls). During and after this deposition andspinning, the ethanol/water azeotropic mixture is evaporating from film29, but due to glycerol's low volatility, no substantial evaporation ofthe glycerol is occurring. This evaporation shrinks thin film 29 andconcentrates the silica content of the sol forming reduced thicknessfilm 33. FIG. 8B shows a reduced thickness sol film 18 obtained aftersubstantially all (about 95% or more) of the ethanol has been removed.This concentrated sol typically gels within minutes or seconds.

Film 33 has an approximately known volume ratio of silica to pore fluidat the gel point. This ratio is approximately equal to the ratio of TEOSto glycerol in the as-deposited sol (with minor changes due to remainingwater, continued reactions and incidental evaporation). To the extentthat the gel is prevented from collapsing, this ratio will determine thedensity of the aerogel film that will be produced from the sol thinfilm.

After gelation, the thin film wet gel 33 comprises a porous solid and apore fluid. The pore fluid may preferably be left in place, although itmay be diluted or replaced by a different fluid (e.g. replace glyceroland water mixture with glycerol). Whether this fluid is identical to theas-gelled fluid or not, the pore fluid that is present during aging issometimes referred to as "aging fluid". Aging is most preferably carriedout in one of the limited volume chambers of the present invention, e.g.for about 1 minute at 130-150° C., although temperatures in the range of25° C. to 200° C., as well as aging times as short as several seconds oras long as one day are also comprehended. It should be noted that thepore fluid changes somewhat during processing. These changes may be dueto continued reactions, evaporation, condensation, or chemical additionsto the thin film.

After aging, wet gel film 33 may be dried without substantialdensification by one of several methods, including supercritical fluidextraction. However, with polyol-based gels, one alternative is to use asolvent exchange to replace the aging fluid with a drying fluid and thenair dry the film 33 from this drying fluid. This drying method uses asolvent exchange to dilute the aging fluid or replace it with adifferent fluid (e.g. use a large volume of acetone to dilute theglycerol and water mixture, thus forming a mixture dominated byacetone). Whether this fluid is identical to the aging fluid or not, thepore fluid that is present during drying is sometimes referred to as"drying fluid". If used, the solvent exchange replaces the aging fluidthat is dominated by the glycerol and its associated high surfacetension with a drying fluid that has a lower surface tension. Thissolvent exchange may preferably be carried out by dispensingapproximately 5-8 mL of acetone at room temperature onto aged thin film18, then spinning the wafer between approximately 250 and 500 rpm forabout 5-10 seconds. In this solvent exchange method, it is preferred toremove nearly all the glycerol before drying. The drying fluid (acetonein this case) is finally allowed to evaporate from the wet gel 18,forming a dry porous dielectric (dried gel).

An alternate method may be used to age and dry, e.g. glycerol-based,films without solvent exchange using a limited volume chamber 32. Anunaged wafer is placed in a temperature-controlled limited volumechamber, preferably at room temperature and ambient pressure. Thechamber remains substantially sealed as the temperature is ramped up,aging the film. After the chamber reaches a temperature at which theglycerol surface tension is low enough such that the aged film issufficiently strong to withstand capillary drying pressures, a processis begun that removes glycerol from the chamber atmosphere. Note thatthe preferred drying temperature, in many applications, is greater thanthe boiling point of glycerol, in which case the chamber should bepressurized before the boiling point is reached. Also, care should betaken that the glycerol in the chamber atmosphere is, especially atfirst, slowly removed. The glycerol in the chamber atmosphere may, e.g.,be removed by bleeding off the pressure, by vacuum pumping, by sweepingthe glycerol off with an annealing gas (e.g. forming gas), or by forcingcondensation on chamber wall 30 (see, e.g. the configuration of FIG.19C). The chamber temperature may be held constant or it may continue tobe raised while the glycerol is being removed (the chamber may be rampedon up to annealing temperature while sweeping the glycerol off with theannealing gas). While some glycerol can be introduced during initialheating to minimize evaporation from the film (until a temperature hasbeen reached where the surface tension of the fluid is sufficientlyreduced), preferably the chamber volume is low enough that evaporationdoes not significantly reduce film thickness even without theintroduction of glycerol during heating.

After sufficient aging, the fluid can be evaporated, e.g. withoutcooling, or the substrate cooled before drying, as condensation does notpose a serious problem after sufficient aging. As the thin film becomespredominately dry (typically within seconds), the temperature shouldpreferably then be increased above the boiling point of both the agingfluid and the drying fluid. This method prevents destructive boiling,yet insures that all fluid is removed.

In order to reduce the dielectric constant, it is preferable todehydroxylate (anneal) the dried gel. This may be done by placing thewafer in a forming gas atmosphere comprised of 10 volume % H2, 90 volume% N2 at atmospheric pressure, and baking at 450 C. for approximately 30minutes. Other anneals can also be used in place of or in conjunctionwith the dehydroxylating anneal.

In accordance with a second, ethylene glycol-containing sol aerogelprocess, mix tetraethylorthosilicate (TEOS), ethylene glycol, ethanol,water, and acid (1M HNO₃) in a molar ratio of 1:2.4:1.5:1:0.042 andreflux for 1.5 hours at 60° C. After the mixture is allowed to cool, thesolution is diluted down with ethanol to a composition of 70% (byvolume) original stock solution and 30% (by volume) ethanol. This ismixed vigorously and typically stored in a refrigerator at 7° C. tomaintain stability until use. The solution is warmed to room temperatureprior to film deposition. A mixture of stock solution and 0.25M NH₄ OHcatalyst (10:1 volume ratio) is combined and mixed. This sol may bedeposited on substrate 26 in the manner described in conjunction withthe glycerol solvent embodiment.

FIG. 9 contains a flow chart of a general method for obtaining anaerogel thin film from a precursor sol according to one embodiment ofthe present invention. Table 5 is a quick summary of some of thesubstances used in this method.

                  TABLE 5                                                         ______________________________________                                        Substance Summary                                                               Ref    Specific  Functional                                                   #  Example Description Preferred Alternates                                 ______________________________________                                        10   Silicon   Semiconductor                                                                            Ge, GaAs, active devices,                               Substrate lower level layers                                                12 Al-0.5% Cu Patterned Al, Cu, other metals, polysilicon                       Conductors                                                                   TEOS Precursor Sol Other silicon-based metal                                   Reactant alkoxides (TMOS, MTEOS,                                               BTMSE, etc.), alkoxides of other                                              metals, particulate metal oxides,                                             organic precursors, and combin-                                               ations thereof                                                              Glycerol Precursor Sol Other polyols, combinations of                          First Solvent glycerol, Ethylene glycol,                                      (Low volatility) 1,4-butylene glycol, 1,5-penta-                               nediol, and/or other polyols.                                               Nitric Acid Precursor Sol Other acids                                         (HNO.sub.3) Stabilizer                                                        Ethanol Precursor Sol Methanol, other alcohols                                 Second Solvent                                                                (High vola-                                                                   tility)                                                                      Ethanol Viscosity Methanol, other alcohols                                     Reduction                                                                     Solvent                                                                      TMCS Surface Hexamethyldisilazane (HMDS),                                      Modification trimethylmethoxysilane, dimethyl-                                Agent dimethoxysilane, phenyl com-                                             pounds and fluorocarbon com-                                                  pounds.                                                                     Ammonium Gelation Ammonia, volatile amine species,                            Hydroxide Catalyst volatile fluorine species, and other                       (NH.sub.4 OH)  compounds that will raise the pH                                 of the deposited sol. Nitric acid                                             and other compounds that will                                                 lower the pH.                                                               As-Gelled Aging Fluid Glycerol, ethylene glycol, other                        Pore fluid  polyols, water, ethanol, other                                      alcohols, combinations thereof.                                             Acetone Drying Fluid Aging fluid, heated aging fluid,                           heptane, isopropanol, ethanol,                                                methanol, 2-ethylbutyl alcohol,                                               alcohol/water mixtures, ethylene                                              glycol, other liquids that are                                                miscible with the aging fluid, yet                                            have lower surface tension than the                                           aging fluid, combinations thereof.                                       ______________________________________                                    

Other ratios of solvent to reactant ratios can be used to providedifferent porosities. FIG. 15 shows the theoretical relationship betweenthe molar ratio of glycerol molecules to metal oxide molecules and theporosity of a nanoporous dielectric for the case where all ethanol isevaporated from the deposited sol. However, the lower porosity gelsrequire care to prevent early gelation. This may comprise pH adjustment,temperature control, or other methods known in the art. In someapplications, it is also permissible to allow ethanol evaporation aftergelation.

Other ratios of solvent to reactant ratios can be used to providedifferent porosities. FIG. 10 shows the theoretical relationship betweenthe molar ratio of ethylene glycol molecules to metal oxide moleculesand the porosity of a nanoporous dielectric for the case where allethanol is evaporated from the deposited sol. However, the lowerporosity gels may require care to prevent early gelation. This maycomprise pH adjustment, temperature control, or other methods known inthe art. In some applications, it is also permissible to allow ethanolevaporation after gelation.

Although this invention has been described in terms of severalembodiments, many steps may be modified or combined within the scope ofthe invention, and other steps can be included to enhance the overallprocess. For example, the initial thin film may be deposited by othercommon methods, such as dip-coating or spray-coating instead ofspin-coating. Likewise, the solvent exchange may use dip coating, spraycoating, or immersion in a liquid or vaporous solvent instead ofspin-coating. When using a high vapor pressure solvent, the wafer may becooled to a temperature lower than the atmosphere, thus promotingcondensation on the wafer. While water might be considered a solvent insuch a process, for discussion purposes in this application, water isnot considered a solvent.

By modifying the mix ratios of polyol and alcohol in the sol-gelprocess, the gel's properties can be changed. One such change is the geltime. Table 6 below shows the results of varying the ethanol to ethyleneglycol ratios in the precursor sol of some sample bulk gels withcatalysts. These gels generally used the same sol mixture as theethylene glycol embodiment except for the ethanol to ethylene glycolratio. Also, in the non-polyol-based mix, the catalyst concentration isdifferent. This non-polyol-based gel used 0.5 M NH₄ OH catalyst in avolume ratio of 1:10, instead of the 0.25 M NH₄ OH used in the others.

                  TABLE 6                                                         ______________________________________                                        Effect of Varying the Ethylene Glycol Content of the Precursor Sol                           Ethanol    Ethylene Glycol                                                                           Content Content Gel Time                  Bulk Example #  (mL) (mL) (minutes)                                         ______________________________________                                        1          61         0            7 to 10                                      (Non-Polyol-Based)                                                            2 36.6 24.4 5 to 7                                                            3 30.5 30.5 2 to 3                                                            4 24.4 36.6 1 to 2                                                            5 0 61 1 to 2                                                               ______________________________________                                    

Another example of modification to the basic method is that, beforedrying (and typically, but not necessarily, after aging), the thin filmwet gel 29 may have its pore surfaces modified with a surfacemodification agent. This surface modification step replaces asubstantial number of the molecules on the pore walls with those ofanother species. If a surface modifier is applied, it is preferable toremove the water from the wet gel 29 before the surface modifier isadded. The water can be removed by immersing the wafer in pure ethanol,preferably by a low speed spin coating as described in the solventexchange in the first process example. This water removal could bebeneficial, because water will react with many surface modificationagents, such as TMCS; however, it is not necessary. With a polyol-basedmethod, surface modification need not be performed to help lessen porecollapse, but it can be used to impart other desirable properties to thedried gel. Some examples of potentially desirable properties arehydrophobicity, reduced dielectric constant, increased resistance tocertain chemicals, and improved temperature stability. Some potentialsurface modifiers that may impart desirable properties includehexamethyldisilazane (HMDS), the alkyl chlorosilanes(trimethylchlorosilane (TMCS), dimethyldichlorosilane, etc.), thealkylalkoxysilanes (trimethylmethoxysilane, dimethyldimethoxysilane,etc.), phenyl compounds and fluorocarbon compounds. The useful phenylcompounds will typically follow the basic formula, Ph_(x) A_(y)SiB.sub.(4-x-y), where, Ph is a phenolic group, A is a reactive groupsuch as Cl or OCHs, and B are the remaining ligands which, if there aretwo, can be the same group or different. Some examples of these phenylsurface modification agents include compounds with I phenolic group suchas phenyltrichiorosilane, phenyltrifluorosilane, phenyltrimethoxysilane,phenyltriethoxysilane, phenylmethylchlorosilane,phenylethyldichlorosilane, phenyldimethylethoxysilane,phenyldimethylchlorosilane, phenyldichlorosilane,phenyl(3-chloropropyl)dichlorosilane, phenylmethylvinylchlorosilane,phenethyldimethylchlorosilane, phenyltrichlorosilane,phenyltrimethoxysilane, phenyltris(trimethylsiloxy)silane, andphenylallyldichlorosilane. Other examples of these phenyl surfacemodification agents include compounds with 2 phenolic groups such asdiphenyldichlorosilane, diphenylchlorosilane, diphenylfluorosilane,diphenylmethylchlorosilane, diphenylethylchlorosilane,diphenyldimethoxysilane, diphenylmethoxysilane, diphenylethoxysilane,diphenylmethylmethoxysilane, diphenylmethylethoxysilane anddiphenyldiethoxysilane. These phenyl surface modification agents alsoinclude compounds with 3 phenolic groups such as triphenylchlorosilane,triphenylflourosilane, and triphenylethoxysilane. Another importantphenyl compound, 1,3-diphenyltetramethyldisilazane, is an exception tothis basic formula. These lists are not exhaustive, but do convey thebasic nature of the group. The useful fluorocarbon based surfacemodification agents include (3,3,3-trifluoropropyl)trimethoxysilane),(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1dimethylchlorsilane, and otherfluorocarbon groups that have a reactive group, such as Cl or OCH₃, thatwill form covalent bonds with a hydroxyl group.

The paragraph above lists some of the typical useful properties for manyconventional applications. However, there are other potentialapplications for nanoporous dielectrics and aerogels that may havedifferent desirable properties. Examples of some other potentiallydesirable properties include hydrophilicity, increased electricalconductivity, increased dielectric breakdown voltage, increasedreactivity with certain chemicals, and increased volatility. This listis not exhaustive. However, it shows that, depending upon theapplication, many different types of properties may be desirable. Thus,it is clear that many other materials that will form covalent bonds withhydroxyl groups are potential surface modifiers that may impart otherpotentially desirable properties.

This invention also comprises using gelation catalysts with theglycerol-based and other polyol-based sols, not just the ethyleneglycol-based sols. This also includes the allowance of other gelationcatalysts in place of the ammonium hydroxide and/or for the gelationcatalyst to be added after deposition. Typically, these alternatecatalysts modify the pH of the sol. It is preferable to use catalyststhat raise the pH, although acid catalysts can be used. Typically, acidcatalysis results in longer processing times and a denser dielectricthan a base catalyzed process. Some examples of other preferred gelationcatalysts include ammonia, the volatile amine species (low molecularweight amines) and volatile fluorine species. When the catalyst is addedafter deposition, it is preferable to add the catalyst as a vapor, mist,or other vaporish form.

Thus, this invention can allow production of nanoporous dielectrics atroom temperature and atmospheric pressure, without a separate surfacemodification step. Although not required to prevent substantialdensification, this new method does not exclude the use of supercriticaldrying or surface modification steps prior to drying. To the extent thatthe freezing rates are fast enough to prevent large (e.g. 50 nm)crystals, it is also compatible with freeze drying. In general, this newmethod is compatible with most prior art aerogel techniques. Althoughthis new method allows fabrication of aerogels without substantial porecollapse during drying, there may be some permanent shrinkage duringaging and/or drying. This shrinkage mechanism is not well understood;however, it behaves in a manner similar to syneresis.

Other examples of modifications involve the reaction atmosphere and/ortemperature. Also coating and gelation need not be performed in the samechamber or even in the same atmosphere. For instance, coating may bedone with a controlled ambient that prevents evaporation of lowvolatility components (particularly at higher temperatures where eventhe low volatility components evaporate more rapidly), or in an ambientthat also prevents evaporation of high volatility components.Additionally, the substrate may have its temperature elevated to speedsurface modification and/or gelation. Also, total pressure and/ortemperature may be varied to further control evaporation rates and/orgel time. Elevated temperature processing is typically performed at noless than 40° C.; however, 50° C. is preferred, and 70° C. is morepreferred. When working at elevated temperatures, care should be taken(e.g., the partial pressures in the reaction atmosphere should be highenough) to prevent solvent boiling.

Although TEOS has been used as a representative example of a reactant,other metal alkoxides may be used either alone or in combination withTEOS or each other to form a silica network These metal alkoxidesinclude tetramethylorthosilicate (TMOS), methyltriethoxysilane (MTEOS),1,2-Bis(trimethoxysilyl)ethane (BTMSE), combinations thereof, and othersilicon-based metal alkoxides known in the art. A sol may also be formedfrom alkoxides of other metals known in the art such as aluminum andtitanium. Some other precursor sols known in the art include particulatemetal oxides and organic precursors. Two representative particulatemetal oxides are pyrogenic (fumed) silica and colloidal silica. Somerepresentative organic precursors are melamine, phenol furfural, andresorcinol. In addition to alternate reactants, alternate solvents mayalso be used. Some examples of preferred alternates for ethanol aremethanol and the other higher alcohols. Other acids may be used as aprecursor sol stabilizer in place of the nitric acid.

An additional modification to the basic process is to allow and/orpromote the formation of moderate sized (15 to 150 monomers permolecule) oligomers in the precursor sol. These larger oligomers mayspeed the gelation process in the deposited sol. A sol containing largeoligomers may have a higher viscosity than a sol with small oligomers.However, as long as the viscosity is stable, this higher viscosity canbe compensated by methods known in the art, such as adjusting solventratios and spin conditions. To help achieve this desired stableviscosity, the oligomerization may need to be slowed or substantiallyhalted before deposition. Potential methods of promoting oligomerizationmight include heating the precursor sol, evaporating solvent, or addingsmall amounts of a gelation catalyst such as ammonium hydroxide.Potential methods of retarding oligomerization might include cooling theprecursor sol, diluting the sol with a solvent, or restoring theprecursor sol to a pH that minimizes condensation and gelation (Nitricacid could be used in conjunction with the ammonium hydroxideexemplified above).

Although the present invention has been described with several sampleembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. An apparatus for processing a wet gel thin filmdeposited on a semiconductor substrate, said apparatus comprising:a bodycapable of substantially enclosing at least a first region of asubstrate surface, said region having a wet gel thin film depositedthereon, said body having a chamber surface capable of being positionedsubstantially adjacent to said substrate surface without contacting saidthin film in said first region, such that a chamber exists between saidchamber surface and said first region, said chamber having a volume lessthan or equal to 5000 times the volume of said thin film in said firstregion; and means for controlling the temperature within said chamber;whereby evaporation of fluid from a wet gel thin film during processingand shrinkage of the film are substantially limited by processing in alimited-volume chamber.
 2. The apparatus of claim 1, wherein saidchamber volume is less than or equal to 1000 times the volume of saidthin film in said first region.
 3. The apparatus of claim 1, whereinsaid chamber volume is less than or equal to 500 times the volume ofsaid thin film in said first region.
 4. The apparatus of claim 1,wherein said body comprises a substantially planar plate and a resilientseal, arranged such that during engagement with a substrate, said sealis interposed between said plate and said substrate and encircles afirst region of the substrate surface.
 5. The apparatus of claim 1,wherein said body comprises a substantially planar plate and a substrateholder capable of being engaged with each other while said substrateholder holds a substrate.
 6. The apparatus of claim 1, wherein saidapparatus processes said substrates in an inverted position.
 7. Theapparatus of claim 6, further comprising means for supplying a layer ofsaid first solvent to said chamber surface.
 8. The apparatus of claim 1,wherein said means for controlling the temperature within said chambercomprise means for controlling the temperature of a substrate andseparate means for controlling the temperature of said chamber surface.9. The apparatus of claim 1, wherein said body further comprises atleast one port into said chamber and means connected to said port foradjusting the atmosphere within said chamber.
 10. The apparatus of claim1, wherein said chamber surface is capable of being positioned such thatsaid substrate surface and said chamber surface are separated by anaverage distance of not greater than 5 mm.
 11. The apparatus of claim 1,wherein said chamber surface is capable of being positioned such thatsaid substrate surface and said chamber surface are separated by anaverage distance of not greater than 1 mm.
 12. The apparatus of claim 1,wherein said wet gel thin film contains a fluid principally comprising apolyol.
 13. An aging chamber for aging a thin film wet gel deposited ona semiconductor substrate, said chamber comprising:a body capable ofsubstantially enclosing at least a first region of a substrate surface,said region having a wet gel thin film wetted by at least a firstsolvent deposited thereon, such that said chamber has an atmosphericvolume adjacent said wet gel thin film which, at a temperature selectedin the range of 80° C. to 200° C., is substantially saturated by anamount of said first solvent equivalent to not greater than 5% of thevolume of said first solvent contained in said wet gel thin film;whereby evaporation of fluid from a wet gel thin film during aging andshrinkage of the film are substantially limited by processing in alimited-volume chamber.
 14. The chamber of claim 13, wherein saidatmospheric volume is saturated by an amount of said first solventequivalent to not greater than 1% of the volume of said first solventcontained in said wet gel thin film.
 15. The chamber of claim 13,wherein said atmospheric volume is saturated by an amount of said firstsolvent equivalent to not greater than 0.5% of the volume of said firstsolvent contained in said wet gel thin film.
 16. The chamber of claim13, wherein said first solvent principally comprises a polyol.